[....] The bosstones
After the voussoirs have been cut, the carving of the bosstones is undertaken. As explained, this vault is built with vertical and tilted bosses; the carving of the vertical ones is done starting from their vertical and horizontal projections (see again the sketch in photo 25). In the next photos, we can see how the students carry out the carving of a bosstone, first by redrawing on a stone block’s flat upper face the design of the horizontal projection (photo26); then, by eliminating the excessive stone, is done the carving in vertical (photo 27-28). Next stage, from the full scale design in elevation results the slope with which should the arms be cut (see in photo 29 and 30 how the upper horizontal plan is taken as the reference plan of the slope angles). Finally, the cutting of the arms with their correct slope is carried out (photo 31).
At this stage, must set off the carving of the tilted bosstones and, right away, the issue of the reason for this type of bosses rises. We know that the Gothic bosses’ shape is determined by their horizontal and vertical projection; nevertheless, along the 15th century start the tilted bosses to appear and with great success; 16th and 17th centuries’ ribbed vaults usually have tilted bosstones, the slope angle varying according to their position.
The only explanation for this type of bosses lies in the beginning of a new geometric tool that enables to obtain a projection on a plan tangent to the vault’s surface in any position. Photo 32 shows the elevation plan of two of these bosses with their axes perpendicular to the tangent plan; nevertheless, we can see how the profile of the subsidiary rib remains distorted, that is to say with its axis always vertical. In photo 33 we see its projection on the tangent plan. [....]
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